In an electric power steering system (EPS) that uses a motor as a driving source, obviating occurrence of a state where application of an excessive motor voltage that exceeds a high limit of voltage that can be output is required, that is, a voltage saturation state is one of issues that should be resolved in order to stably generate assist force to achieve favorable steering feeling.
The rotation angular velocity of the motor (motor speed) is proportional to a voltage applied to the motor. As the motor speed increases, a larger voltage command value is computed. However, in an EPS that uses an in-vehicle battery as a main power supply, the voltage that can be output from a driving circuit thereof has a high limit. Therefore, at the time of steering at a high speed, at the time when impact force acts on steered wheels, or the like, a voltage saturation state may occur with an increase in the motor speed. When a torque ripple occurs due to voltage saturation and then the torque ripple propagates to a vehicle cabin in form of sound or vibrations, there is a possibility that a steering feeling deteriorates.
Conventionally, as measures against the above described steering feeling deterioration, there has been disclosed a method of limiting a current command value computed as a target value for power assist control. For example, see Patent Document 1.
Usually, in the EPS, torque generated by the EPS is controlled through current control. A control device for the EPS first computes a current command value (q-axis current command value) as a target value of motor torque that should be generated in order to apply assist force, and secondly, in order to cause an actual current value (q-axis current value) to follow the current command value, computes a voltage command value corresponding to the deviation therebetween. Thirdly, in order to apply a voltage indicated by the voltage command value to the motor, motor control signals for operating a driving circuit are generated.
By decreasing a current limiting threshold with an increase in the motor speed, it is possible to suppress an increase in the deviation between a current command value and an actual current value, which occurs in a motor high-speed rotation range, and an increase in voltage command value accordingly. In this way, an attempt to suppress voltage saturation is made.
However, there is also an occurrence factor of voltage saturation, other than an increase in the motor speed. The output voltage of the motor driving circuit, which is utilized as a voltage applied to the motor, varies depending on variations in power supply voltage. As the power supply voltage decreases, the output voltage of the motor driving circuit decreases and then the voltage applied to the motor decreases. Thus, as the power supply voltage decreases by a large amount, a voltage saturation state more easily occurs. In addition, even at the time when the motor output is increased, voltage saturation easily occurs due to a voltage drop resulting from conduction of a large current.
Note that most of motor control devices that supply driving electric power to a motor on the basis of a voltage stepped up by a step-up circuit are configured to, when the power supply voltage is decreased, suppress the step-up control to attempt to reduce a load on the step-up circuit. For example, see Patent Document 2. Therefore, when the power supply voltage is decreased, the voltage that can be output significantly decreases. As a result, the motor control device provided with a step-up circuit has a feature that voltage saturation further easily occurs as compared with a control device with no step-up circuit.